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Atmospheric Sciences & Global Change
Research Highlights

August 2015

Demystifying Aerosols

PNNL researchers show that higher-resolution climate models can provide more accurate information on important particle-cloud interactions

atmospheric rivers model
Aerosol particle mysteries. PNNL researchers demonstrated that higher-resolution climate models can more accurately depict the important interactions between clouds and tiny aerosol particles from pollution, dust, and soot. The aerosols, not always as visible as in this view over a city, have an impact on clouds and climate. Image courtesy Wikimedia Commons License. zoom Enlarge Image.

Results: How global climate models simulate something so small as the interaction between tiny atmospheric particles and clouds can have a surprisingly big impact on results. A research team led by Pacific Northwest National Laboratory used innovative modeling and diagnostic methods to demonstrate that higher-resolution models more accurately depict the interactions between particles suspended in air, called aerosols, and the complex processes within clouds. Going further, the team identified the physical mechanisms behind the improved calculations. The results could guide the development of next-generation climate models.

"Our analyses indicate that aerosol effects on clouds change as the model detail increases," said Dr. Po-Lun Ma, the PNNL atmospheric scientist who led the study reported in Geophysical Research Letters.

"However, even in very high resolution models, there can be large differences between model results and observations," said Ma. "Such differences suggest that resolution increases are not enough, and the physical treatment of the aerosols and cloud features requires further improvement to reduce the uncertainties associated with aerosol-cloud interactions in climate models."

Why It Matters: Climate models involve millions of calculations on such things as air flow, heat transfer, and movement of moisture through the atmosphere. Researchers examine every process to ensure it functions as close to real-world observations as possible so that results are reliable for understanding climatic changes. Unfortunately, the exact mechanisms of interactions between clouds and tiny aerosol particles from pollution, dust, and soot remained largely a mystery. This study showed that higher resolution climate models tend to more realistically depict how droplets form a cloud and how precipitation develops. The results of such models more closely match estimates based on real-world data.

Methods: Scientists used the Community Atmosphere Model version 5.2 (CAM5) to zero in on aerosol-cloud interactions. To facilitate direct comparison between model simulations and satellite observations, they prescribed realistic meteorological fields such as winds and temperature while allowing the model to use equations to calculate how clouds and aerosols interact. They then examined four scenarios of varying degrees of detail. The approach allowed them to compare model results with real-world data at particular times and locations. 

What's Next? Scientists will continue to work on ways to improve the way aerosol-cloud interactions are portrayed in climate models. The PNNL team is currently applying the approach, which grew out of the Aerosol Climate Initiative, to other types of simulations, so that future high-resolution climate models will solve the mystery surrounding aerosol-cloud interactions.

Acknowledgments

Sponsors: The U.S. Department of Energy's Office of Science, Biological and Environmental Research supported this project via the Earth System Modeling Program, with additional support from the National Science Foundation.

Facilities: The National Center for Atmospheric Research's Wyoming Supercomputing Center provided computational resources. The scientists obtained data for the analysis from the National Center for Atmospheric Research, NASA Langley Research Center, and Colorado State University.

Research Team: Po-Lun Ma, Philip Rasch, Hailong Wang, Steven J. Ghan, Richard C. Easter, Jr., and William I. Gustafson, PNNL; Minghuai Wang, Nanjing University, China; Xiaohong Liu, University of Wyoming; and Yuying Zhang and Hsi-Yen Ma, Lawrence Livermore National Laboratory

Research Area: Climate & Earth Systems Science

Reference: Ma P-L, PJ Rasch, M Wang, H Wang, SJ Ghan, RC Easter, WI Gustafson, Jr., X Liu, Y Zhang, and H-Y Ma. 2015. "How Does Increasing Horizontal Resolution in a Global Climate Model Improve the Simulation of Aerosol-Cloud Interactions?" Geophysical Research Letters 42(12): 5058-5065. DOI: 10.1002/2015GL06418


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